Light-emitting diodes (LEDs) can be used for lighting systems with advantages of higher energy efficiency, longer life, smaller size, etc. To produce sufficient brightness, multiple LEDs coupled in series, in parallel or in serial-parallel combinations can be applied.
FIG. 1 shows a conventional LED circuit 100. The circuit 100 includes LED strings 102, 104 and 106, a direct current (DC) power supply 160, a DC/DC converter 110, a selection circuit 120, and linear regulators 122, 124 and 126. Each of the LED strings 102, 104 and 106 includes serially coupled LEDs.
The DC/DC converter 110 converts a DC voltage VDC from the DC power supply 160 to an output voltage VOUT for driving LEDs. Due to variation in LED manufacturing, currents flowing through the LED strings 102, 104 and 106 may not be identical. The linear regulators 122, 124 and 126 are used to regulate the currents flowing through the LED strings 102, 104 and 106 in a linear mode, respectively. The linear regulators 122, 124 and 126 also send feedback signals indicative of forward voltage drops of the LED strings 102, 104 and 106 to the selection circuit 120, respectively. The selection circuit 120 can select a feedback signal having a maximum level (maximum feedback signal) from the feedback signals. The maximum feedback signal can be used by the DC/DC converter 110 to regulate the output voltage to a level no less than the maximum forward voltage drop of the LED strings 102, 104 and 106.
However, due to the power dissipation in the linear regulators 122, 124 and 126, the circuit 100 may have relatively low power efficiency.
FIG. 2 shows a conventional circuit 200. The circuit 200 includes a DC power supply 260, a DC/DC converter 210, LED strings 202, 204 and 206, switching regulators 222, 224 and 226, diodes 262, 264 and 266, inductors 272, 274 and 276, and switching controller 232, 234 and 236. The switching regulators 222, 224 and 226 can be used to regulate and balance currents flowing through the LED strings 202, 204 and 206 in a switching mode, respectively. The switching controllers 232, 234 and 236 respectively control the switching regulators 222, 224 and 226 to operate in the switching mode. The diode 262 and the inductor 272 are used for averaging the current flowing through the LED string 202. Similarly, the diode 264 and the inductor 274 are used for averaging the current flowing through the LED string 204; the diode 266 and the inductor 276 are used for averaging the current flowing through the LED string 206.
However, multiple switching controllers and switching regulators in FIG. 2 may lead to a relatively high circuit cost and a relatively complex circuit structure.